A technique of optimizing the speed change ratio of a CVT of a vehicle according to the slope of a road is disclosed in, for example, Japanese Unexamined Patent Publication No. 61-220938 and Japanese Unexamined Patent Publication No. 62-113956.
In Japanese Unexamined Patent Publication No. 61-220938, the speed change ratio of the CVT is controlled according to a required output (throttle opening) and the slope of a road. More precisely, the speed change ratio is controlled in proportion to the product of a throttle opening and the slope of a road.
In Japanese Unexamined Patent Publication No. 62-113956, the slope of a road is calculated according to the torque, acceleration, etc., of the vehicle to thereby control the speed change ratio of the CVT based on the calculated result. More precisely, transmission speed change maps are changed according to the slope of a road.
These prior techniques, however, have the following problems:
With the first technique, the speed change ratio of the CVT is controlled in proportion to the product of a throttle opening and the slope of a road. Since this product becomes zero when a throttle valve is completely closed, the CVT may be shifted up when an accelerator pedal is released. If the accelerator pedal is released before a curve on an uphill road, the CVT will be shifted up, and if the accelerator pedal is pushed down at the end of the curve, the CVT will be shifted down, to thereby cause transmission-gear-ratio hunting.
With the second technique, although the slope of a road is calculated to change transmission speed change maps, these maps may cause the CVT to be shifted up when the accelerator pedal is released, resulting in transmission-speed change ratio hunting on a curved uphill road.
To solve these problems, an object of the present invention is to provide a control apparatus for a vehicle having a continuously variable transmission, capable of maintaining a proper transmission ratio even when an accelerator is released on a curved uphill road and thereby avoiding transmission-speed change ratio hunting.